Projection welding is well known in the art. Typically a projection is created on the first of two metal objects of similar composition by milling, stamping, or otherwise. The first metal object is biased into contact with the second metal object and an electric current is passed between the two metal objects. The electric current is passed from a point on the projection on the first of the two metal objects into a contact point on the second metal object until such time as the metal comprising the projection and the contact point liquefy. The current is interrupted, the metals cool and the two metal objects remain jointed by solidified metal.
The projection point on the first metal object serves two purposes. The first purpose served is that of a convenient contact point and method of localizing the electric current to a desired location. Since the projection is created in anticipation of joining the two objects the projection may be located at will by a designer.
The projection also serves a second purpose in controlling the flow of welding current. The flow of low frequency electric current in metals is generally regarded as a mass flow phenomenon. The greater the cross-sectional area of current flow in any given situation, the lower the current density. The lower the current density the less localized heating occurs relative to the electric current.
In projection welding an electrical path is presented comprising a projection on the first metal and a contact point on a plane located on the second metal. The projection presents a relatively small cross-sectional area to the flow of electric current. The small cross-sectional current path between the two metals provides a circuit path of relatively high resistance between the objects. Because of the higher resistance within the projection-contact interface the generation of heat created by the weld cycle is concentrated at the interface.
During a projection weld, heating occurs within the projection and contact point and the metals within each liquefy. The projection to a substantial degree collapses. To maintain current flow and complete the weld the two objects must be biased together to maintain contact during liquefaction. As the base metal surrounding the projection come into contact with the second metal the weld cycle is interrupted. The metals mix and subsequently re-solidify resulting in a continuous metallic connection between the first and second metals.
The power source used in projection welding varies according to the application. Where the thickness of each metal is significant, a transformer may be connected in series with the interface. Where the metals are thin adequate power may be provided through the discharge of a capacitor.
Power control in projection welding is offered through the use of timers and through the presence of internal impedance within the welding circuit. To offer sufficient power to the weld site the internal impedance of the power supply and external weld circuit must be low relative to the impedance of the interface between the two metals to be welded. If, on the other hand, the impedance of the power supply and external weld circuit is too low then excessive power is delivered to the weld site and blow-out at the weld interface may occur.
In most cases excessive current are prevented by matching the weld circuit to the weld interface. Thicker materials require larger transformers with lower impedances. Matching the weld circuit to the interface allows for a controlled rate of rise of temperature at the weld interface and a controlled zone of liquefaction. A timer is used to terminate the weld cycle at an appropriate instant when the volume of liquefaction is adequate.
As previously mentioned projection welding has been typically limited to similar metals. Similar metals offer the greatest opportunity for mixing of metals within the zone of liquefaction at a weld point and coincidentally with weld strength. Because of the economy and strength of projection welding a need exits for a method of applying projection welding technology to the joining of dissimilar materials. Efforts, in the prior art, to use projection welding of dissimilar metals have been only partially successful, producing results of widely varying weld strengths.